Two-way ball valves are well known in the art, and have long been used to control fluid flow from a source (upstream) to a destination (downstream). Multiple port valves (three or four way) similarly control fluid flow from a source to one or more destinations depending on the type of valve (three or four way; "T" or "L" shaped ports through the ball). A typical diverter ball valve would have three ports, one upstream and two downstream. A diverter ball valve would thus have two operating positions: a first position would direct fluid from a source to one downstream port, and the second position would direct the fluid from the source to the other downstream port. The upstream port would be open in both positions, and one of the two downstream ports would be open and the other closed depending on the operating position. An annular sealing subassembly is typically provided about each port.
Ball valves are generally classified as either of the trunnion-mounted variety or the floating ball variety. Trunnion-mounted ball valves seal primarily on the upstream seat, whereas floating ball valves seal primarily on the downstream seat. Valve sealing effectiveness is generally a direct function of sealing forces: the higher the forces, the better the seal. Trunnion-mounted ball valves typically have lower operating torque and thus poorer sealing effectiveness than a floating ball valve. Floating ball valves are thus often preferred because they generally perfect a better seal than trunnion-mounted ball valves, although they normally do require a higher operating torque.
In a trunnion-mounted ball valve, the ball either may be physically joined to, or may be an integral part of, a trunnion which extends from the top and bottom of the ball. In either case, the trunnions (stems) are retained by the valve body and define the axis on which the ball is rotated. A typical trunnion-mounted ball valve is disclosed in U.S. Pat. No. 3,732,885. Force on the ball generated by line pressure is retained by the trunnions acting against the valve body. Both the upstream and downstream seats are typically energized with a biasing force against the ball, using substantial biasing pressure generated by a spring or similar device. As the valve is required to seal against higher line pressures, additional upstream seat force against the ball is created. For all practical purposes, most trunnion-mounted ball valves effectively seal at high pressure only on the upstream side of the ball. An example of a sealing subassembly for trunnion-mounted ball valves is disclosed in U.S. Pat. No. 4,147,327.
Trunnion-mounted ball valve seats are thus generally retained in position by the valve body, are preloaded with a spring force to maintain sealing engagement with the ball, perfect a low pressure seal, and assist the line forces in high pressure upstream sealing. These seats are typically metallic or a combination of metallic and nonmetallic elements. Trunnion-mounted ball valves with metallic seating rings are disclosed in U.S. Pat. Nos. 4,262,688; 4,318,420 and 4,386,756. U.S. Pat. No. 4,519,412 discloses a trunnion-mounted ball valve with a sealing subassembly adapted to receive a sealant should a leak occur. Trunnion-mounted ball valves having a seat ring including an elastomeric material for perfecting the seal against the ball are disclosed in U.S. Pat. Nos. 3,752,178 and 4,273,309. Special tools are typically required to install and replace seats in a top entry trunnion-mounted ball valve.
Floating ball valves have a ball that is free to float in any direction with respect to the body. The ball is typically restrained only by the seats. The ball is forced against the downstream seats by line pressure when in the closed position. Depending on the nature of the valve, the upstream seat may float against the ball in a manner similar to a trunnion-mounted ball valve. Alternately, the upstream seat may be in a permanent position with respect to the valve body, in which case it separates from the ball as the ball moves downstream to seal. Sealing the third or fourth ports of a floating ball valve presents additional difficulties, since the ball motion is now not limited to linear movement between a single upstream port and a single downstream port.
Although the dimensional movement of the ball in a floating ball valve is minimal, this movement is critical and essential to the proper sealing of the valve. The line pressure creates a substantial force on the ball that is proportional to the bore size (actually the effective seat sealing diameter) of the valve. This substantial sealing force is not available in a trunnion-mounted ball valve because the ball is rigidly mounted by the trunnions.
To perfect an effective seal, floating ball valve seats are typically fabricated from an elastomeric material and are preloaded with a force against the ball both upstream and downstream. A floating ball valve with elastomeric seats is disclosed in U.S. Pat. No. 3,721,425. A "free floating" elastomeric seal is disclosed in U.S. Pat. No. 4,535,525, and a temperature sensitive valve body/seat seal is disclosed in U.S. Pat. No. 4,658,847. A floating ball valve with a deformable thin walled sheet metal seal is presented in U.S. Pat. No. 4,502,663.
Ball valves with elastomeric seats generally do not provide long life service when subjected to erosive fluids. Rigidly mounted metallic seats in either a trunnion-mounted or floating ball valve may provide increased life compared to elastomeric seats, and are frequently specified by valve users. Rigid metallic seats in conventional floating ball valves encounter significant problems, however, when used in an abrasive environment, such as oil and gas well fluids having sand and hard minerals entrained therein. When a sand grain or other hard foreign particle gets trapped between the ball and seat, the seat is forced away from the ball so that the separation of the ball and the seat allows the valve to leak. Equally important, at this time the rigid seat supports the ball at only two points, one where the foreign matter is trapped and the other directly across the seat from this point (180.degree. apart). This localized force on the rigid seat is extremely high, and in almost all cases will deform the seat and/or the ball (gauld one or both surfaces), thereby creating a permanent leak and causing valve failure.
U.S. Pat. No. 4,326,752 discloses one effort to increase the life of a trunnion-mounted ball valve by incorporating a metal scraper ring as part of the seat assembly. The metal ring removes scale and foreign matter from the ball and protects the elastomeric seal from abrasion. A problem with this design is that the wiper seal will wear quickly when operating in a bath of abrasive fluids because of its small cross section and high localized forces against the ball. Once the wiper seal has worn, its effectiveness is diminished and foreign matter will abrade the seat and ball, and the valve will no longer seal.
Those skilled in the art have also recognized that the manufacturing costs of ball valves is inherently related to the required machining accuracy of the valve seat components, and that the overall operating cost of the valve is a function of the time and expense associated with repairing the valve, as described in U.S. Pat. No. 4,441,524. Nevertheless, prior art ball valves have failed to satisfy customer needs for many applications, particularly when handling abrasive fluids.
The disadvantages of prior art ball valves, both trunnion-mounted and floating ball types, have caused some users to specify poppet-type valves. In the poppet-type valve, the seat sealing surface are typically exposed to the flow stream at all times, and are thus subject to abrasion and yield a short service life. Poppet valves also have restricted ports, with the fluid being forced to flow around a complex geometry including the poppet piston itself. This flow arrangement causes two major problems: one, there is substantial restriction to the flow across the valve, and two, the combination of a turning flow stream and the reduced port size results in excessive erosion to the valve body and internal parts.
The disadvantages of the prior art are overcome by the present invention, and an improved ball valve is hereinafter disclosed which offers substantially increased service life over prior art valves when handling abrasive and corrosive fluids.